Fouling refers to the undesirable attachment of organic matter, biomolecules, and microbes on submerged surfaces. Fouling diminishes the performance of devices involving these submerged surfaces and is considered the bottle-neck issue for various applications in the biomedical industry, food processing, and water treatment. Membrane fouling is a serious problem in processes such as reverse osmosis (RO) water desalination, industrial water treatment, juice concentration, and hemodialysis. On a separation membrane e.g., a RO membrane, a biofilm matrix forms and acts as a secondary membrane, which adversely affects the separation capabilities of the membrane. The biofilm causes a significant drop in trans-membrane pressure resulting in a decline in permeate water flux with time. To keep the permeate flux at optimum levels, higher operating pressures are required. Furthermore, the physical damage of the membrane caused by the biofilm results in increase in the salt passage (i.e., reduced salt rejection). To combat the adverse effects of biofouling, membranes must be frequently cleaned with harsh chemicals; chemical treatment of the membrane, in turn, results in shorter membrane life. The cumulative effect of all these factors causes an enormous increase in operating cost of the desalination process.
Developing anti-biofouling surfaces is an active area of research; it encompasses many applications, such as coating for ship hulls, biomedical implants, devices, and biosensors, food packaging, and industrial and marine equipment. However, this technique has not been explored fully in the case of separation membranes, such as RO membranes.
Zwitterionic materials have been explored for their ultra-low protein adsorption and outstanding antifouling properties, and have been extensively investigated as hydrophilic or fouling-resistance modifiers. Some relevant examples include the synthesis of sulfobetaine methacrylate (SBMA), 2-methaeryloyloxy ethyl phosphorylcholine (MPC), and carboxybetaine methacrylate (CBMA). These zwitterionic coatings have been used as hydrogels, polymer brushes, double-hydrophilic copolymer, and polyelectrolytes.
To synthesize zwitterionic coatings, several innovative techniques have been developed. For example, self-assembled monolayers (SAMs), solution polymerization and solvent evaporation, and atom transfer radical polymerization (ATRP) have been used. These methods have several limitations. For example, SAMs require specific surface functionality and, therefore, are limited to gold substrates only. ATRP methods require the use of harsh solvents, which are not compatible with delicate RO membranes. Furthermore, the use of solvents in solution polymerization and ATRP would not only damage the delicate RO membrane substrate, but may also lead to high surface roughness, which would diminish anti-biofouling properties of the coated surface.
One example of the synthesis of a polyzwitterionic membrane involved synthesis of amphiphilic precursor poly(vinylidene fluoride)-graft-N,N-dimethylaminoethyl methacrylate (PVDF-g-PDMAEMA) by ATRP, followed by blending with PVDF to prepare flat membranes. The membranes were then reacted in tetrahydrofuran (THF) solution with 1,3-propanesultone (1,3-PS) or 3-bromopropionic acid (3-BPA) to yield SBMAs or carboxybetaines acrylic acetates (CBAAs), respectively. Z. Yi et al. J. Membr. Sci. 385-386 (2011) 57-66. By this method, only 70% of PDMAEMA was quaternized in 48 h for the 1,3-PS, while only 62% of PDMAEMA was quaternized for the 3-bromopropionic in 96 h. Lower conversion (i.e., quaternization) of the PDMAEMA precursor may be the result of both synthesis by ATRP which may render some of the PDMAEMA inaccessible to the quaternizing agent. Alternatively, or in addition, the lower diffusion kinetics of liquid-phase reactions, as compared to gas-phase reactions, may play a role.
Another example involved the preparation of a co-polymer film by reacting 2-(dimethylamino)ethyl methacrylate (DMAEMA) and ethylene glycol dimethacrylate via initiated chemical vapor deposition (iCVD). The film was then treated with 1,3-PS to convert the amphiphilic precursor DMAEMA to zwitterions (polysulfobetaines (pSBs)). R. Yang et al. Chem. Mater. 2011, 23, 1263-1272. In contrast to most bulk solution-phase polymerization and solvent evaporation methods, this process resulted in a high concentration of zwitterionic moieties (up to 90%) on the top surface (˜10 nm), which is desirable for antifouling applications. The high surface zwitterionic content was attributed to the diffusion-limited gas-phase reaction at the surface.
In addition, a zwitterionic poly(carboxymethylbetaine) (PCMB) brush on fused quartz has been described. T. Kondo et al. Colloids and Surfaces B: Biointerfaces 100 (2012) 126-132. This polymer was made by surface initiated ATRP of CMB monomer and displayed some desirable antifouling characteristics.
There exists a need for an efficient, reliable, environmental-friendly, solvent-less method of depositing chemically-stable antifouling coatings on reverse osmosis membranes.